ISTANBUL (Reuters) - People had to swim to safety in the flooded streets of Istanbul on Tuesday after heavy rain and storms brought chaos to Turkey's largest city and caused shipping traffic on oil and grain routes through the Bosphorus Strait to be suspended for some hours.

Many roads, bridges, tunnels and metro stations were flooded and cars submerged in torrential downpours that followed two hours of lightning flashes across the city.

Television footage showed people swimming in the streets and being rescued by boats from streets and flooded houses.

Heavy rainfall and poor visibility caused traffic through the Bosphorus, an important international shipping lane for oil and grain, to be suspended from 8.30 a.m. (0530 GMT) to midday, shipping agents said.

Turkish authorities also closed the Dardanelles strait to tankers longer than 200 meters after an oil tanker ran aground, but also reopened the strait at noon.

"What we are experiencing is a natural disaster," Transportation Minister Ahmet Arslan was quoted as saying by broadcaster CNN Turk.

Prime Minister Binali Yildirim said there were no casualties.

CNN Turk said more than 6,700 bolts of lightning hit the city in a space of two hours and cited the Istanbul municipality as saying 65 kg of rain per square meter had fallen since the morning - the equivalent of 6.5 cm.

The Istanbul Ferry Company (IDO) was cited by broadcaster NTV as saying it had canceled some of its cruises, while public transportation on land also slowed to a halt.

Istanbul Governor Vasip Sahin said citizens should be wary of using their personal cars unless necessary, and he warned of more heavy rain after 2 p.m. (1100 GMT).

Water Management minister Veysel Eroglu was quoted by CNN Turk as saying the rainfall would continue until Wednesday.

(Reporting by Can Sezer; Additional reporting by Tuvan Gumrukcu in Ankara; Editing by Richard Balmforth)